A few notes about

TRANSFORMERS

 

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Every model railroad needs some sort of power supply.  The first power supply for a model railroad was contained in a box referred to as a transformer.  To be technical it was a bit more than that but in fact it was more that than anything else by weight and that was its main function.  The box also contained a speed control, a way of reversing the train, some sort of circuit breaker, and a few things more.

 

The transformer remains the key element of a power supply as shown below.  The other components, if included in the circuit, remain inside the same box and included in other parts of the circuit.  Only the parts that comprise the power supply are shown here.

 

 

 

 

Building a remote control  transistor throttle for model train control starts with a power supply.  Even a DCC has its own internal power supply.

 

Transformer 

 

The Transformer, as far as model railroading is concerned, is an AC device used to change dangerous 112 volts house current to a safer 18 volts with a much lower current level. 

 

From 2500 watts to something closer to 20 or less.

 

voltage levels and/or isolate circuits. 

 

    current is transferred from one wire (primary)  to other (secondary)

    a magnetic core is used to increase the efficiency of this transfer

extra connections added for flexibility

      center tap primary

      primary wire has extra connection

      twin coil primary for 220 vs. 110 AC input

secondary tapped or split like primary

     sometimes series for voltage

     sometimes parallel for current

     sometimes unequal or multiple for various voltages for other circuits

     secondaries electrically isolated inside

 

beware the autotransformer

it is possible to have a single primary tapped to provide secondary

it works but dangerous – no isolation from outside current

 

benefits of transformer

isolates dangerous power levels

change voltage level

step up

step down

 

types of transformers   

          Plug-in wall transformers – hand for small amounts of current

         Chassis mount used where more power is desired

         PC mount – in between plug-in and chassis mount

        Toroidal a specialized doughnut-shaped ring

                 More expensive, more efficient,

  

   

     Wall transformer            Standard transformer      toroid transformer

 

 

 

    

 

Selecting the correct transformer for a project can be tricky

      First, look at specs

           Voltage

            Amps,

             Volt amps

       A 18v transformer measures 24v AC

                DC volts * 1.4 = AC volts

                Transformer drops as load applied

                 Transformers get warm at rated voltage and need ventilation

       Outside factors

            Each stage of power supply

                      AC house current can vary

                      Fuse has tiny voltage drop

                      Transformer output voltage drops under load

                       Diodes of rectifier add their amount

                               Typical drop is .4v

                       Voltage regulators require (3) volt drop

                              Over this contributes to overheating

                              Less may cause erratic performance

                       Further drop in throttle circuits

                              Junctions each add their drop in final stage

                               Contacts of reversing relay etc all add their drop

                       Layout wiring resistance

       Best bet

            Start with  external power supply

            Build circuit

            Measure required values

            Then build power supply

             Test and verify as required

      

         

 

 

 

  

                         

           

 

 

 

 

    Transformers

     

       

    

 

 

 

 

  

   

Fuses

   Fuse on a circuit board

No fuse will protect any solid state component from damage

      Proper size transformer will minimize risk

      But will provide margin of safety

      Should not be ignored        

 Bi-metallic strip

       Different metals expand at different rates

           Current warms spring

           Spring trips at rated value

           Grasshopper fuse

                 Simplest and cheapest

                Can be adjusted Grasshopper fuse

            Pre-set

                 More precise and adjustable

                 Limited number of trips

             Metallic strip fuse

                 Most common

                  One use only

             Sealed breaker

                   Trips at rated current

                   Automatic resets

 

 

 

    

     Fuse on a circuit board     

    

 

 

 

    

 

   

 

     

AC to DC conversion 

   

Rectifier, capacitor, and resister   

 

     A diode allows current to flow in on direction only

           Can be used AC to Convert to DC

            Two can improve flow

      Four diodes make up rectifier

             Converts AC to DC more efficiently

 

      C1 is required for electronics

            Prevents DC from dipping to zero

            Holds closer to AC levels

           

    

      R1 and LED1 serve two purposes

            Warn user that circuit is active and in use

                   Imply AC may be present

            Quickly drain capacitor

                  Prevent nasty shocks when working with circuit

 

 

    

 

 

 

   

Power Regulator 

 

Regulator   

The is a black box

      Takes many forms

       Single component to built up circuits

            

 

    

        This supply is good for about 0.1 amps for very small projects

 

 

   This supply is more general for about a half amp continuous

Or about a full amp for short periods due to the small heat sink. 

    

Switching power supplies

    Mainstay of computers

    Smaller, cooler, much more efficient,

    more complex

This dual power supply

    illustrates use of toroidal transformer

    Details Not in scope of this article

    Suffice to say that this regulator supplies two voltages,

     On for twelve volts and five amps.

 

 

 

 

 

 

 

Power Supply  

This Power cage (formerly a steel 1950’s hi-fi amplifier) supplies the entire electrical needs for the North River Railway.  Note the fuse in the upper right corner and the switching voltage regulators in the second bay of the cage.  The three transformers in the bottom bay proved throttle power.

 

    

 

          

   

 

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Model Railroading is fun in Connecticut.
Bob Van Cleef, MMR

Last update  08/02/2012